Camshaft adjusters for varying the valve opening and closing timing of the gas exchange valves in internal combustion engines, particularly in automotive vehicles, are known per se. By the variation of valve timing it is intended to optimize different operating parameters, particularly engine torque, idle stroke quality, fuel consumption as well as pollutant emission as a function of the engine speed.
In engines comprising an inlet camshaft and an outlet camshaft, for example in the DOHC (double overhead camshaft) engine having two overhead camshafts, the outlet and inlet operations can be adjusted relative to each other as a function of the engine speed by camshaft adjusters mostly configured as electrohydraulic adjusters. For this purpose, it is generally the inlet camshaft that is adjusted through a certain angle relative to the crankshaft through which both camshafts are driven. In the region of the upper gas exchange dead point, this causes a variation of the overlap of the inlet and outlet operations during which both inlet and outlet valves are open at the same time. During this overlap a scavenging process takes place in which fresh gas is fed in and residual gas is expelled. At the same time, due to backflow processes of exhaust gas, an “internal” exhaust gas recirculation that changes the residual gas mass in the compression chamber takes place. The exhaust gas recirculation reduces the combustion temperature and thus contributes to the reduction of NOx pollutant emission. On the other hand, an excessive residual gas mass leads to a deterioration of the idling quality (erratic idling behavior). Therefore, during idle running, an adjustment to a “retarded” inlet closing is executed with the help of the camshaft adjuster, so that, through a small valve overlap, the idling quality is improved or a high degree of idling quality assured.
An adjustment to retarded inlet closing is likewise performed at high engine speeds. Because of the retarded inlet closing, a vibrating air column that develops particularly at high speeds in the engine cylinder leads to a gas dynamic recharging effect in the combustion chamber that results in an enhancement of engine performance. In contrast, at medium engine speeds, the camshaft is adjusted for to an “advance” inlet closing in order to achieve a better cylinder filling that leads to a higher torque.
A camshaft adjustment, particularly with a view to reducing pollutants, can be appropriate even in engines having only one camshaft, for example in OHV (overhead valves) engines with a bottom camshaft or in SOHC (single overhead camshaft) engines with an overhead camshaft. However, due to the fact that the valve overlap remains constant, the setting of valve opening and closing timing is always a compromise.
Other known hydraulic valve lash adjusters automatically adjust a valve lash resulting from thermal expansion and wear of the transmission elements that transmit a cam lift produced by the camshaft to the gas exchange valves, so that a perfect operation of the internal combustion engine is guaranteed. These adjusting elements are generally configured as hydraulic tappets comprising a control valve that is biased in closing direction by a spring. This means that, through the force of the spring, the control valve is closed for most of the time. Thus, these lash adjusters, as quasi rigid elements, transfer a cam lift directly to the gas exchange valves.
Valve lash adjusters in which the control valve is biased in opening direction are being increasingly used. Such lash adjusters, configured, for example, as RSHVA (reverse spring hydraulic valve adjusters) or as NOLA (normally open lash adjusters) are known, for instance, from U.S. Pat. No. 4,054,109, U.S. Pat. No. 5,758,613 and JP 61 185607 A. In these lash adjusters, the control valve is held in open position during the cam base circle phase by the force of the spring. Because the lash adjuster can only be closed by hydrodynamic and hydrostatic forces arising from an oil stream flowing from a high pressure chamber to a low pressure chamber of the tappet with commencement of the cam lobe, the lash adjuster at first produces an idle stroke before the actual valve lift begins. In particular, the idle stroke reliably prevents a negative valve lash and can be put to use for compensating mechanical shortcomings, particularly in the case of camshafts in vehicles having adjustable cams and/or camshafts.
The idle stroke of such lash adjusters can influence the valve overlap and thus also the opening and closing timing of the gas exchange valves. Normally, the idle stroke is largest at the idling speed of the engine and decreases more or less linearly with rising speed. The relatively large idle stroke during idle running reduces the valve overlap, so that the quantity of residual gas formed during combustion in the cylinder combustion chamber is reduced. This has a positive effect on the idling quality of the engine with respect to a stable idling motion.
Camshaft adjusters serve to modify the valve overlap in different engine speed ranges. Idle stroke elements likewise modify the valve overlap. Disadvantageously, the thermodynamic potentials offered by camshaft adjustment and valve lash adjustment with help of the idle stroke have not been exhaustively exploited in the past. In particular, no method of variable valve timing is known so far in which an influence of hydraulic valve lash adjusters with idle stroke on the valve timing in internal combustion engines equipped with camshaft adjusters is adequately regulated. In internal combustion engines with camshaft adjustment and idle stroke elements, an offsetting of valve timing having an unfavorable effect in certain operational situations can take place.